I've had lots of practice adding points for my crypto class. However I've run into a situation where I need to subtract two points for decryption:
Pm + kPb - nb(kG) = Pm
Where Pm is the plaintext, Pb is participant b's public key, nb is participant b's private key, G is a base point in the elliptic group Ep(a,b), and k is a random positive integer chosen by participant a.
I am able to compute nb(kG) and kPb, but I'm unsure how to subtract the two. How is this done?